Fluid filtration method, fluid filtered thereby, lithographic apparatus and device manufacturing method

ABSTRACT

A method for filtering a fluid to obtain a fluid having a known purity is described. The fluid is filtered with a filtration system, and upstream of a final filtration stage of the filtration system, a purity of the fluid is measured. A purity of the fluid filtered by the filtration system is determined by correcting the measured purity with a filtration behavior of the final filtration stage. In an embodiment, the fluid comprises an ultra pure water for use as an immersion liquid in a lithographic apparatus.

This application is a continuation of U.S. patent application Ser. No.11/062,764, filed on Feb. 22, 2005, now U.S. Pat. No. 7,378,025, whichis herein incorporated by reference in its entirety.

FIELD

The present invention relates to a fluid filtration method, a fluidfiltered thereby, a lithographic apparatus and a method formanufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application publication WO 99/49504, herebyincorporated in its entirety by reference. As illustrated in FIGS. 2 and3, liquid is supplied by at least one inlet IN onto the substrate,preferably along the direction of movement of the substrate relative tothe final element, and is removed by at least one outlet OUT afterhaving passed under the projection system. That is, as the substrate isscanned beneath the element in a −X direction, liquid is supplied at the+X side of the element and taken up at the −X side. FIG. 2 shows thearrangement schematically in which liquid is supplied via inlet IN andis taken up on the other side of the element by outlet OUT which isconnected to a low pressure source. In the illustration of FIG. 2 theliquid is supplied along the direction of movement of the substraterelative to the final element, though this does not need to be the case.Various orientations and numbers of in- and out-lets positioned aroundthe final element are possible, one example is illustrated in FIG. 3 inwhich four sets of an inlet with an outlet on either side are providedin a regular pattern around the final element.

In the solutions as described above and further below, an immersionliquid is applied. A purity of the immersion liquid should be high, toavoid absorption and/or scattering of radiation by impurities in theimmersion liquid. Furthermore, the purity of the immersion liquid shouldbe high to avoid a deposit of small quantities of one or more substancescomprised in such impurities on a surface of an optical element (e.g., alens) of the projection system such as the final element indicated inFIG. 5. Due to the extremely high requirements on a projection by aprojection system, a layer having a thickness of only a few moleculesmay deteriorate the projection to an unacceptable extent.

To obtain an immersion liquid having a purity sufficiently high to meetthe above requirements, a filtration system, such as a chemicalfiltration system, is used. A problem however is that purityrequirements as posed on the immersion liquid are so high, thatmeasurement devices known in the art are or may not able to measure sucha high purity thus making it difficult to verify whether or not thefiltered liquid meets the purity requirements.

SUMMARY

Accordingly, it would be advantageous, for example, to provide a fluid,such as an immersion liquid, having a high, known purity.

According to an aspect of the invention, there is provided a method forfiltering a fluid and determining a purity of the filtered fluid,comprising:

filtering the fluid with a filtration system;

measuring a purity of the fluid upstream of a final filtration stage ofthe filtration system to obtain a measured purity; and

determining a purity of the fluid filtered by the filtration system bycorrecting the measured purity with a filtration behavior of the finalfiltration stage.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising:

projecting a patterned beam of radiation onto a substrate via a liquid,the liquid having been filtered by:

filtering the liquid with a filtration system;

measuring a purity of the liquid upstream of a final filtration stage ofthe filtration system to obtain a measured purity; and

determining a purity of the liquid filtered by the filtration system bycorrecting the measured purity with a filtration behavior of the finalfiltration stage.

According to an aspect of the invention, there is provided an ultra purewater having a known purity, the water having been filtered by:

filtering the water with a filtration system;

measuring a purity of the water upstream of a final filtration stage ofthe filtration system to obtain a measured purity; and

determining the known purity of the water filtered by the filtrationsystem by correcting the measured purity with a filtration behavior ofthe final filtration stage.

According to an aspect of the invention, there is provided alithographic projection apparatus arranged to project a pattern from apatterning device onto a substrate via an immersion liquid, theimmersion liquid having been filtered according to the method accordingto the invention.

According to an aspect of the invention, there is provided alithographic projection system arranged to project a pattern from apatterning device onto a substrate via a liquid, comprising a filtrationsystem configured to filter the liquid and having:

a final filtration stage; and

a measurement system configured to measure a purity of the liquidupstream of the final filtration stage to obtain a measured purity andto determine a purity of the liquid filtered by the filtration system bycorrecting the measured purity with a filtration behavior of the finalfiltration stage.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts a further liquid supply system for use in a lithographicprojection apparatus; and

FIG. 6 depicts a filtration system configured to filter a fluidaccording to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or DUV radiation).

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables and/or supportstructures may be used in parallel, or preparatory steps may be carriedout on one or more tables and/or support structures while one or moreother tables and/or support structures are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another immersion lithography solution with a localized liquid supplysystem solution which has been proposed is to provide the liquid supplysystem with a liquid confinement structure which extends along at leasta part of a boundary of the space between the final element of theprojection system and the substrate table. Such a solution isillustrated in FIG. 5. The liquid confinement structure is substantiallystationary relative to the projection system in the XY plane thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). See, for example, U.S. patent application Ser. No.10/844,575, hereby incorporated in its entirety by reference. A seal istypically formed between the liquid confinement structure and thesurface of the substrate. In an embodiment, the seal is a contactlessseal such as a gas seal.

Referring to FIG. 5, reservoir 10 forms a contactless seal to thesubstrate around the image field of the projection system so that liquidis confined to fill a space between the substrate surface and the finalelement of the projection system. The reservoir is formed by a liquidconfinement structure 12 positioned below and surrounding the finalelement of the projection system PL. Liquid is brought into the spacebelow the projection system and within the liquid confinement structure12. The liquid confinement structure 12 extends a little above the finalelement of the projection system and the liquid level rises above thefinal element so that a buffer of liquid is provided. The liquidconfinement structure 12 has an inner periphery that at the upper end,in an embodiment, closely conforms to the shape of the projection systemor the final element thereof and may, e.g., be round. At the bottom, theinner periphery closely conforms to the shape of the image field, e.g.,rectangular though this need not be the case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the liquid confinement structure 12 and the surface of thesubstrate W. The gas seal is formed by gas, e.g. air or synthetic airbut, in an embodiment, N₂ or another inert gas, provided under pressurevia inlet 15 to the gap between liquid confinement structure 12 andsubstrate and extracted via first outlet 14. The overpressure on the gasinlet 15, vacuum level on the first outlet 14 and geometry of the gapare arranged so that there is a high-velocity gas flow inwards thatconfines the liquid.

FIG. 6 shows a schematic filtration system comprising a first filtrationstage F1, a second filtration stage F2 and a third, final filtrationstage F3. A fluid, in this example a liquid such as water, is led intothe filtration system at a filtration input IN, the fluid beingsuccessively filtered by the filtration stages F1, F2 and F3. Oncefiltered by these filtration stages, the fluid is drained from thefiltration system at a filtration system output OUT. The filtrationstages F1, F2 and F3 may comprise any type of filter, such as anabsorption filter or a chemical filter. As outlined above, a fluidhaving a high purity is required, which brings forward the problem thatmeasurement of such a high level of purity may be difficult with ameasurement system according to the state of the art or may involveextremely costly and/or time consuming measurement techniques and/ormeasurement devices. In the filtration system according to FIG. 6, ameasurement system M is shown which measures a purity of the fluidupstream of the final filtration stage F3. In the example depicted inFIG. 6, a measurement probe MP of the measurement system is in contactwith the fluid upstream of the final filtration stage F3. Alternativelyto the use of a probe in contact with the fluid upstream of the finalfiltration stage, a sample could (e.g. periodically) be taken from thefluid upstream of the final filtration stage F3 and be provided to themeasurement system. Thus, the measurement system provides a measuredpurity of the liquid before having been filtered by the final filtrationstage F3. A filtration behavior of the final filtration stage F3 isdetermined, as will be explained in more detail below, and with thisfiltration behavior of the final filtration stage F3, the measuredpurity is corrected to determine a purity of the fluid filtered by thefiltration system.

The behavior of the final filtration stage F3 can be determined in anumber of ways, some examples thereof will be described below. Thefiltration behavior of the final filtration stage may be determined byfiltering an amount of the fluid by means of the final filtration stageor with the filtration system comprising the final filtration stage. Apurity of the fluid before having been filtered by the final filtrationstage is determined as well as a purity of the fluid after having beenfiltered by the final filtration stage. Such a purity may be determinedby concentrating the fluid to obtain a concentrated fluid, e.g. byheating the fluid such that a part of it is vaporized. By vaporizing apart of the fluid, a remainder thereof may comprise a more concentratedlevel of impurities, thus a lower purity, which eases a measurement of apurity thereof. It may be possible to perform the concentration on (asample of) the fluid before having been filtered by the final filtrationstage, as well as on (a sample of) the fluid after having been filteredby the final filtration stage. Alternatively or additionally, samplescan be taken for off-line analysis in a dedicated laboratory, making useof analysis techniques which are more accurate than in-line analysismethods and instruments that can be used on-line during filtration.Additionally or alternatively, it is possible to determine thefiltration behavior of the final filtration stage from a theoreticalmodel, making use of, for example, filter characteristics such as filtermolecule structures, pore sizes, molecular structures of the impuritiesfound in the fluid, etc.

The correcting of the measured purity with a filtration behavior of thefinal filtration stage can be performed by any suitable correctionoperation such as a mathematical subtraction or a mathematical divisionor any other correction operation. If the behavior of the finalfiltration stage comprises a filtration factor of 100 then the purity ofthe fluid filtered by the filtration system is determined by dividingthe measured impurity levels by a factor of 100. Alternatively, it couldbe possible to measure impurity levels upstream, add a known, cleanfluid, and correct measured impurity levels by a dilution factor.

The measurement system may comprise any purity measurement systemsuitable for measuring a purity of the to be filtered fluid, such as aconductivity meter, a TOC (Total Organic Carbon) analyzer, a silicaanalyzer, a particle counter, etc.

The filtration as described here may be unselective, i.e. filtering anyimpurity of a broad range of impurities from the fluid. Alternatively,the filtration may be selective, i.e. filtering a specific type ofimpurity of a specific group of impurities from the fluid.

The method as described above makes it possible to determine a purity ofthe liquid as filtered by the filtration system, and in particular bythe final filtration stage F3, even in a situation where the purity ofthe fluid filtered by the filtration system is that high that it cannotbe reliably measured with a state of the art measurement system.

An embodiment is however not restricted to a filtration to obtain anhighly pure fluid. It may also be used in any other applicationproviding one or more other advantages, such as in a situation where ameasurement system having a sensitivity which is sufficiently high to beable to detect the purity of the fluid filtered by the final filtrationstage F3 is costly, in a situation where such measurement system is notavailable at a site of filtration, in a situation where a qualifiedoperator to operate such measurement system is not present, etc.

Further, according to an embodiment, it is possible to determine adegradation of the filtration system, and in particular a degradation ofthe final filtration stage F3 in a manner as will be described below.Commonly, a filtration stage, such as the filtration stage F3, has acertain life span. In case that the final filtration stage F3 comprisesa mechanical filter, the filter will get clogged by impurities which arefiltered out of the fluid by the filtration stage. In case that thefinal filtration stage comprises a chemical filter, such chemical filterwill get saturated by substances which have been filtered out of thefluid by the filtration stage. In such a chemical filter, a filtrationsubstrate comprises a plurality of molecule structures which are able tochemically bind an impurity molecule or impurity molecular structure inthe to be filtered fluid. The more of these molecules and/or molecularstructures have been filtered by the filtration stage, the lesseffective the filter becomes, as each impurity molecule or impuritymolecular structure that is retained in the filtration stage occupies aposition in a molecular structure of the filtration substrate which isable to chemically bind an impurity. The less “free” molecularstructures on the substrate which are able to bind impurities are leftover, the less effective the filtration stage becomes. Such a situationwhere an effectiveness of the filtration stage shows a significant decayis commonly referred to as a breakthrough of the filtration stage. Abreakthrough of the final filtration stage F3 can be predicted byintegrating the measured purity (which has been measured upstream of thefinal filtration stage F3) during a total filtration time of the finalfiltration stage. By integrating the measured purity of the fluidprovided to the final filtration stage, a total amount of impuritiesoffered to the filtration stage F3 can be determined. In a chemicalfilter, a total amount of impurities which may be retained by the filteris known, as chemical properties of the filtration substrate, and inparticular its impurity retaining capacity, are known. Similarly, in anabsorption filter, a total amount of impurities which may be retained bythe filter before clogging or other saturation phenomena start toappear, is known. Thus, the integrated measured purity (which provides ameasure for the total amount of impurities filtered by the filter) maybe compared with a maximum impurity dose of the filtration stage, suchthat a warning can be generated and/or the final filtration stage F3 canbe replaced before a breakthrough, clogging, etc. occurs. This methodmay be particularly advantageous with a chemical filter, as a chemicalfilter does not allow for measurement of its performance by conventionaltechniques, such as measuring a pressure difference between a fluidupstream and a fluid downstream of the respective filter. In thechemical filter, molecule groups are exchanged, which implies that noclogging will be observed when the chemical filter reaches its end oflife. Thus, the embodiment to predict a breakthrough of the filter, andin particular of the final filtration stage, may be particularlyadvantageous with a chemical filter or chemical filtration stage.

In an embodiment, the method described above may advantageously beapplied to filter an immersion fluid, in particular immersion liquid foruse in a lithographic apparatus. As described, a purity required by thelithographic apparatus is that high, that a purity of the immersionfluid when filtered by the filtration system may not be measurable withmeasurement devices and measurement techniques as known in the state ofthe art. In an embodiment, the immersion fluid comprises an immersionliquid such as a water, the water having advantageous opticalcharacteristics making it suitable for immersion purposes. In anembodiment, the immersion liquid comprises ultra pure water, for examplebeing pH neutral (i.e. having a pH of 7), preferably being low in ioniccontamination (i.e. comprising a resistivity of at least 18 MOhm*cm, forexample being low in metals (order of magnitude mid to low ppt range),for example being low in organic contamination (order of magnitude TOClow ppb range or lower), for example being low in silica contamination(order of magnitude low ppb range of lower), for example being low ingaseous contamination (order of magnitude mid ppb range or lower) and/orfor example being low in particle contamination (less than a few hundredparticles per liter in a smallest measurable size, in an embodimentbeing in an order of magnitude of a smallest lithographic image detailor smaller). In an embodiment, the filter is constructed for filteringsilica compositions such as silicates. Silica compositions, and inparticular silicates, tend to easily deposit on a surface of a finaloptical element (e.g., lens) of the projection system. A layer of suchdeposit having a thickness of only three molecules may cause asubstantial degradation of an image quality of the projection system,comprising the final optical element. The projection system isconsidered a highly costly part of the lithographic apparatus. Thus, adegradation of its characteristics by a deposit on the final opticalelement of impurities contained in the immersion fluid should be avoidedas replacement or cleaning of the projection system may be costly and/ortime consuming, which implies a use of an extremely pure immersionfluid. With the method according to an embodiment of the invention, apurity of such an extremely pure immersion fluid may be determined.Although filtration and purity determination of an immersion liquid isconsidered an advantageous embodiment of the invention, one or moreembodiments of the invention are however not restricted thereto but maybe applied to filter any fluid at any (high or low) purity level.

According to an embodiment, there is provided a device manufacturingmethod comprising projecting a patterned beam of radiation onto asubstrate via an immersion liquid (e.g. making use of the liquid supplysystem 12 as shown and described with reference to FIG. 5, or any othersolution such as described with reference to FIGS. 2-4), the immersionliquid having been filtered according to the method described above.Also, according to an embodiment, there is provided a lithographicprojection apparatus arranged to project a pattern from the patterningdevice onto the substrate via the immersion liquid, the immersion liquidhaving been filtered according to the method described above.

The method for filtering a fluid may be applied to any type of fluid,including a liquid as well as a gas. The fluid may thus comprise aliquid and/or a gas. Impurities comprised in the fluid may comprise anytype of impurities, however as described above the method according toan embodiment of the invention is advantageously used in combinationwith a chemical filter thus to filter chemical impurities. Nevertheless,the method may equally well be applied with any other type of impurity.Where in this document the term a high level of purity or the term ahigh purity is used, this is to be understood as a low level ofimpurities, i.e. a low level of contamination, contaminating substancesor any other unwanted substances. The impurities may comprise any typeof impurity, including a gaseous, liquid and/or solid impurity. Theimpurity may include any kind of substance.

In European Patent Application No. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table. In a preferred embodiment, the apparatus,method and/or computer program product as described herein is applied toa single stage/table lithography apparatus.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCD's), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath or only on a localized surface area of the substrate. A liquidsupply system as contemplated herein should be broadly construed. Incertain embodiments, it may be a mechanism or combination of structuresthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise a combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets that provideliquid to the space. In an embodiment, a surface of the space may be aportion of the substrate and/or substrate table, or a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.The liquid supply system may optionally further include one or moreelements to control the position, quantity, quality, shape, flow rate orany other features of the liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A filtration system configured to filter a liquid, the filtrationsystem comprising: a final filtration stage; and a measurement systemconfigured to measure a purity of the liquid upstream of the finalfiltration stage to obtain a measured purity and to determine a purityof the liquid filtered by the filtration system by correcting themeasured purity with a filtration behavior of the final filtrationstage.
 2. The filtration system according to claim 1, wherein themeasurement system is configured to determine the filtration behavior ofthe final filtration stage prior to determining the purity of the liquidby: filtering an amount of the liquid by means of the final filtrationstage; concentrating the quantity of the liquid to obtain a concentratedliquid; and measuring a purity of the concentrated liquid.
 3. Thefiltration system according to claim 1, wherein the measurement systemis configured to determine the filtration behavior of the finalfiltration stage from a theoretical model prior to determining thepurity of the liquid.
 4. The filtration system according to claim 1,wherein the measurement system is configured to predict a breakthroughof the final filtration stage by integrating the measured purity duringa total filtration time of the final filtration stage.
 5. The filtrationsystem according to claim 1, wherein the liquid comprises ultra purewater.
 6. The filtration system according to claim 1, wherein the finalfiltration stage comprises a chemical filter.
 7. The filtration systemaccording to claim 1, wherein the final filtration stage is constructedto filter silica compositions.
 8. A system, comprising: a lithographicapparatus to project a patterned beam of radiation onto a substrate; aliquid supply system configured to supply a liquid to a space within thelithographic apparatus; and a filtration system configured to filter theliquid, the filtration system comprising: a final filtration stage; anda measurement system configured to measure a purity of the liquidupstream of the final filtration stage to obtain a measured purity andto determine a purity of the liquid filtered by the filtration system bycorrecting the measured purity with a filtration behavior of the finalfiltration stage.
 9. The system according to claim 8, wherein themeasurement system is configured to determine the filtration behavior ofthe final filtration stage prior to determining the purity of the liquidby: filtering an amount of the liquid by means of the final filtrationstage; concentrating the quantity of the liquid to obtain a concentratedliquid; and measuring a purity of the concentrated liquid.
 10. Thesystem according to claim 8, wherein the measurement system isconfigured to determine the filtration behavior of the final filtrationstage from a theoretical model prior to determining the purity of theliquid.
 11. The system according to claim 8, wherein the measurementsystem is configured to predict a breakthrough of the final filtrationstage by integrating the measured purity during a total filtration timeof the final filtration stage.
 12. The system according to claim 8,wherein the final filtration stage comprises a chemical filter.
 13. Thesystem according to claim 8, wherein the final filtration stage isconstructed to filter silica compositions.
 14. A system, comprising: aliquid supply system configured to supply a liquid to a space within alithographic apparatus; and a filtration system configured to filter theliquid, the filtration system comprising: a final filtration stage; anda measurement system configured to measure a purity of the liquidupstream of the final filtration stage to obtain a measured purity andto determine a purity of the liquid filtered by the filtration system bycorrecting the measured purity with a filtration behavior of the finalfiltration stage.
 15. The system according to claim 14, wherein themeasurement system is configured to determine the filtration behavior ofthe final filtration stage prior to determining the purity of the liquidby: filtering an amount of the liquid by means of the final filtrationstage; concentrating the quantity of the liquid to obtain a concentratedliquid; and measuring a purity of the concentrated liquid.
 16. Thesystem according to claim 14, wherein the measurement system isconfigured to determine the filtration behavior of the final filtrationstage from a theoretical model prior to determining the purity of theliquid.
 17. The system according to claim 14, wherein the measurementsystem is configured to predict a breakthrough of the final filtrationstage by integrating the measured purity during a total filtration timeof the final filtration stage.
 18. The system according to claim 14,wherein the liquid comprises ultra pure water.
 19. The system accordingto claim 14, wherein the final filtration stage comprises a chemicalfilter.
 20. The system according to claim 14, wherein the finalfiltration stage is constructed to filter silica compositions.
 21. Asystem, comprising: a liquid supply system configured to supply a liquidto a space; a liquid confinement structure configured to confine theliquid in the space; and a filtration system configured to filter theliquid, the filtration system comprising: a final filtration stage; anda measurement system configured to measure a purity of the liquidupstream of the final filtration stage to obtain a measured purity andto determine a purity of the liquid filtered by the filtration system bycorrecting the measured purity with a filtration behavior of the finalfiltration stage.